1. Field of the Invention
This invention relates to a wide deviation phase-locked loop tracking filter circuit. More particularly, the present invention relates to a novel tracking filter circuit which provides an output signal having ultra-low phase noise.
2. Description of the Prior Art
Tracking filters were known heretofore and are employed to clean up input signals which are degraded by thermal noise and other types of interference signals. The prior art tracking filters employed a single phase-locked loop to produce the desired output signal. The prior art phase-locked loops were employed to track the slowly varying frequency of the input signal within the loop bandwidth of the phase-locked loop. For frequencies outside of the loop bandwidth, the phase noise of the output signal is identical to the noise of the free running loop oscillator. As the bandwidth of the input signal to the prior art tracking filters becomes wider the output signal phase noise increases.
Heretofore the output signal phase noise from wide deviation tracking filters would degrade as the frequency deviation of the phase-locked loop increased. The wider bandwidth causes the tracking filter to have a lower circuit Q at the loop oscillator which results in increased phase noise.
It would be desirable to provide a tracking filter circuit which is capable of providing output signals having ultra-low phase noise and a tracking filter circuit which does not increase phase noise degradation of the signal with increased input signal bandwidth.